1. Field of the Invention
The present invention relates to a rubber composition and a pneumatic tire and, more particularly, to a rubber composition having improved heat resistance and a pneumatic tire using said rubber composition.
2. Description of the Related Art
Heretofore, a reinforcing layer composed of a rubber composition alone or a composite of a rubber composition and another material such as a fiber is disposed at side wall portions of a pneumatic tire to enhance the rigidity of the side wall portions. However, when the temperature of the rubber composition increases to 200xc2x0 C. or higher, for example, in use of the pneumatic tire in a condition in which the pressure inside the pneumatic tire (hereinafter referred to as the inner pressure) is reduced by a puncture or the like, i.e., in a run-flat condition, the rubber composition tends to show rupture of crosslinked portions obtained by vulcanization or of the polymer itself forming the rubber component. Due to an increase in deformation of the caused by a decrease in modulus side wall portion, heat generation of the rubber composition is accelerated and/or the limit of failure of the rubber lowers. As the result, the tire is damaged in a relatively short time.
To increase the time before the tire is damaged to as long as possible, the modulus of the rubber composition may be increased as much as possible by modifying the formulation of the rubber composition or the heat generation in the rubber composition itself may be suppressed by decreasing tan xcex4 of the rubber composition as much as possible by modifying the formulation of the rubber composition. However, improvement by modifying the formulation has limit. Further, when the amounts of rubber reinforcing layers and/or bead fillers are increased to ensure durability of a desired level or higher, ride comfort under normal running condition and noise level deteriorate and the weight of the tire increases.
An object of the present invention is to provide a rubber composition having improved heat resistance and a pneumatic tire produced using this rubber composition, more particularly, a pneumatic tire exhibiting improved durability using this rubber composition as a rubber composition for side wall reinforcing layer and/or a rubber composition for bead portion reinforcing rubber filler
As the result of intensive studies by the present inventors on various chemicals used in formulations to improve heat resistance of the rubber composition, it was found that the heat resistance of a rubber composition can be remarkably improved when specific compounds are used in the formulation. The present invention has been completed on the basis of this knowledge.
The object of the present invention can be achieved in accordance with (1) to (18) described in the following.
(1) A rubber composition having, in a curve exhibiting a change in dynamic storage modulus during elevation of temperature, an intersection of an extrapolation line A of a portion in which the dynamic storage modulus shows an approximately linear change before a rapid decrease at temperatures higher than 100xc2x0 C. and an extrapolation line B of a portion in which the dynamic storage modulus rapidly decreases, at a temperature of 170xc2x0 C. or higher.
(2) A rubber composition described in (1), which comprises sodium 1,6-hexamethylenedithiosulfate dihydrate.
(3) A rubber composition described in any of (1) and (2), wherein the amount of sodium 1,6-hexamethylenedithiosulfate dihydrate is 1 to 10 parts by weight per 100 parts by weight of a rubber component.
(4) A rubber composition described in any of (1) to (3), which comprises a compound A having two or more ester groups in one molecule.
(5) A rubber composition having, in a curve exhibiting a change in dynamic storage modulus during elevation of temperature, a difference xcex94Exe2x80x2 between the maximum value and the minimum value of the dynamic storage modulus at a temperature between 180 and 200xc2x0 C. of 2.5 MPa or less.
(6) A rubber composition described in (5), which comprises a compound A having two or more ester groups in one molecule.
(7) A rubber composition described in any of (4) and (6), wherein the compound A is an acrylate or a methacrylate.
(8) A rubber composition described in any of (4), (6) and (7), wherein the compound A is a polyfunctional ester of a polyhydric alcohol and acrylic acid or methacrylic acid.
(9) A rubber composition described in any of (4), and (6) to (8), wherein the polyhydric alcohol forming the compound A is at least one compound selected from the group consisting of tetramethylolmethane, trimethylolpropane and polymers of these compounds.
(10) A rubber composition described in any of (4), and (6) to (9), wherein the polyhydric alcohol is trimethylolpropane or a dimer of tetramethylolmethane.
(11) A rubber composition described in any of (4), and (6) to (10), wherein the amount of the compound A is 0.5 to 20 parts by weight per 100 parts by weight of a rubber component.
(12) A pneumatic tire comprising side wall portions reinforced with a rubber reinforcing layer, wherein a rubber composition for the rubber reinforcing layer comprises the rubber composition described in any of (1) to (11).
(13) A pneumatic tire comprising bead fillers for which the rubber composition described in any of (1) to (11) is used.
(14) A pneumatic tire described in any of (12) and (13), which is a run-flat tire.
(15) A rubber composition for side reinforcing layers and/or bead fillers comprising sodium 1,6-hexamethylenedithiosulfate dihydrate.
(16) A rubber composition described in (15), which comprises a compound A having two or more ester groups in one molecule.
(17) A rubber composition for side reinforcing layers and/or bead fillers which comprises a compound A having two or more ester groups in one molecule.
(18) Use of sodium 1,6-hexamethylenedithiosulfate dihydrate in side reinforcing layers and/or bead fillers of a pneumatic tire.